1. Field of the Invention
The present invention relates to a method of operating a device for controlling electric actuators with optimum actuation current distribution.
2. Description of Related Art
More specifically, the present invention may be used to advantage, though not exclusively, in the control of solenoid valves controlling intake and exhaust of a vehicle internal combustion engine, e.g. an engine featuring a VVA (Variable Valve Actuation) valve control system, to which specific reference is made in the following disclosure purely by way of example. The method according to the invention, in fact, may be applied to other types of electric actuators, such as solenoid valves of ABS devices and similar, electronic injectors, etc.
More specifically, in the VVA system, an oil chamber, connected to an electrically controlled hydraulic valve (solenoid valve), is interposed between the mechanical cam and the intake and exhaust valve. When the solenoid valve is closed, actuation of the intake and exhaust valve follows the movement of the mechanical cam; and, when the solenoid valve is open, the cam partly empties the oil chamber and so disconnects actuation of the intake and exhaust valve from the mechanical cam. By appropriately controlling the solenoid valve, it is therefore possible to achieve any valve lift profile within the maximum envelope permitted by the movement of the mechanical cam, and dynamic, precise control of the amount of air and burnt gases inside the combustion chamber.
European Patent EP 0 924 589 filed by the present Applicant relates to a device for controlling electric actuators, and which, in particular, controls a number of inductive electric actuators connected in parallel between the same power line and the same ground line by respective controlled electronic switches.
More specifically, the power line is connected by an external cable to a voltage source, e.g. the vehicle battery (typically supplying 13.5 V), and is also connected to a capacitor bank.
In the above control device, the electric actuator actuation current profile is PWM (Pulse Width Modulation) regulated. More specifically, the controlled electronic switch connecting the respective electric actuator to the power line is PWM controlled when the current circulating in the electric actuator is to be maintained about a predetermined actuation value.
The mean current demand of the electric actuator can roughly be shown to be supplied directly by the voltage source—in the example described, by the vehicle battery—over the external cable, while the rest of the current (“ripple” current) is supplied by the capacitor bank according to the equation:ICrms=√{square root over (I)}Lrms2−ILmean2where:    ICrms is the effective value of the current circulating in the capacitor bank;    ILrms is the effective value of the current circulating in the electric actuator; and    ILmean is the mean value of the current circulating in the electric actuator.
The electric actuators can be activated fully independently by the control device, so that, in certain operating conditions, actuation may overlap or be completely superimposed in time.
In which case, the current demands of simultaneously activated electric actuators may be summed, thus resulting in a considerable increase in the effective current ICrms required of the capacitor bank.
Since the capacitor bank is designed on the basis of the effective current circulating in it, the increase in effective current ICrms therefore calls for using large, high-cost capacitors.